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ABOUT THIS IMAGE:

How do astronomers weigh a star? These images help tell the story.

In the image at top, left, astronomers discovered a subtle brightening
of a star [located within the box] due to the effect of gravitational
microlensing. This phenomenon occurs when a foreground star, in this case
a dim red star, passes in front of a much more distant star and amplifies
its light. Astronomers were engaged in a large-scale search for microlensing
events in the halo of our Milky Way galaxy. They were looking in the
direction of the Large Magellanic Cloud, a satellite galaxy of our Milky
Way. The image was taken in February 1993 with the 50-inch telescope at the
Mount Stromlo Observatory in Australia. The box represents the field of
view of NASA's Hubble Space Telescope.

When astronomers used the Mount Stromlo telescope to observe the same
region almost a year later, the background star had returned to its normal
brightness. The foreground star  the "natural lens" that had magnified
the background star  had moved away. The ground-based telescope's vision,
however, was not keen enough to resolve the stars separately.

So, astronomers used the sharp vision of the Hubble telescope to
resolve the stars as two separate objects. The foreground star is red,
and is in our galaxy's halo. The background star is blue, and is in the
Large Magellanic Cloud. The image was taken on July 11, 2002.

Seeing the two stars allowed astronomers to calculate the foreground star's
distance from Earth, which is 1,800 light-years. They already knew that the
background star is 170,000 light-years away, the distance to the Large
Magellanic Cloud. Knowing the distances to both stars then allowed
astronomers to calculate the foreground star's mass, which is one-tenth the
mass of the Sun.